currently we cannot parse multi-instance data

[giuseppec]

OpenML dataset with ID 1438 has multi-instance observation. In arff files this is denoted by a 'relational' variable-type which is currently not supported by any arff reader in R.
The header looks like:

 @attribute bag relational 
    @attribute y numeric 
    @attribute x numeric 
    @attribute z numeric 
@end bag 

see also openml/OpenML#230

[berndbischl]

We currently throw an informative error. This is non-critical and can be extended later.

[berndbischl]

and that we throw that informative msg is for now unit tested, and we have doced this in readARFF. We are good for now